and on the Memoir of Eider. 241 



yet not so strongly marked as I have seen them with Mr 

 Cuthberfs /gths of an inch metals, having' an aperture of 

 i^ths of an inch; a striking proof, in my mind, of the supe- 

 riority of the reflecting principle, for the advantage in point 

 of power is three to one in favour of the refractor. However, 

 every artist will eternally insist that there is nothing equal to 

 his own leather, be it what it may. This set comes up to a 

 power equal to that of a y' 5 th of an inch lens without any for- 

 cing, and the vision is, with this amplification, certainly equal in 

 point of distinctness to that of any single or compound mag- 

 nifier, and superior in the article of achromatism, field of view, 

 &c. The said set of object-glasses are in the possession of 

 Mr Pritchard, Picket Street, Strand. 



H. I have lately received an intimation from M. Chevalier, 

 that he expects to effect a still greater reduction of the focus 

 of his object-glasses, by forming an achromatic system, by 

 combining two over-corrected object-glasses with a common 

 lens, according to the plan laid down by me in my paper on 

 facilitating the manufacture of engyscopic objectives. See this 

 Journal, No. viii. p. 244. 



I. It is not my intention to cuter into any controversy here 

 concerning the invention of achromatic object-glasses, which 

 being a very valuable one. has of course plenty of fathers to 

 it. It must be obvious, on the most superficial consideration, 

 that the object-glasses of microscopes are merely modifications 

 of those of telescopes, therefore, there was nothing to be in- 

 vented about them. 



We are perpetually annoyed with new applications and 

 adaptations of old things, which are falsely termed new inven- 

 tions ; thus it is customary to style a steam-boat, a steam-car- 

 riage, steam-gun, a cotton-mill, &c. new inventions, though 

 they are only so many ingenious and valuable applications of 

 a steam-engine, highly creditable to those who have brought 

 iluin into use. 



Euler is undoubtedly the original adapter of achromatic ob- 



UCt-glasses t<> tin compound microscope. Their use as niag- 



niliiTs, as eye-pieces, as well a?, for forming tin- image of the 



solar microscope, was, 1 think, originally pointed out by my- 



ilim the first treatise I wrote upon them; bul all these ap- 



